This invention relates to engine-driven welding power supplies and more particularly to D.C. welding power supplies using an engine-driven, multiphase alternator providing current to a multiphase thyristor rectifier circuit.
Welding operations often take place on construction sites, at remote sites, and in other locations where a self-contained power supply is advantageous. In such situations, it is common for a gasoline or diesel engine to be used as a prime mover driving an alternator or generator that supplies welding power. One type of power supply uses a gasoline or diesel motor to drive an induction-type, three-phase alternator having a three-phase alternating current output. Three-phase alternating current is a standard form for transmitting power in which alternating current (a sine wave) of about 50 or 60 hertz is provided on three conductors and, sometimes, a neutral. The three-phase power is created by a rotating magnetic field in the alternator that causes current to flow in stationary windings connected to form three phases. The three phases are 120 degrees out of phase with one another and are conventionally referred to as phases A, B and C. By "120 degrees out of phase" it is meant that phase B is one-third of a cycle behind phase A and phase C is one-third of a cycle behind phase B. The phase order is determined by the direction of rotation of the magnetic field which is in turn determined by the direction of rotation of the driving prime mover. An efficient, well-understood type of power transmission is provided.
The three-phase alternating current output is rectified by use of thyristors in a rectifier bridge. The thyristors are fired in a manner which allows the control of welding parameters such as welding current magnitude.
The control of the thyristors in the rectifier circuit requires one to synchronize the firing signal with the cycles of the alternator voltage which forms the input to the thyristor rectifier bridge. This can be difficult because the firing of the thyristors themselves, the operation of the welder and other elements in the system introduce spikes, noise and false transitions into what would ideally be a smooth sine wave thyristor input.
In the past, synchronizing has sometimes been accomplished by obtaining a synchronizing signal from the power output lines of the alternator itself. The above-described spikes, noise, false transitions and other problems require conditioning of the synchronizing signal to eliminate false synchronizing and mistiming. Another approach provides one dedicated synchronizing winding in the alternator for each phase. Such an approach is expensive as it requires three synchronizing windings for a three-phase power supply. Such an approach also requires conditioning of the output from the synchronizing windings because the noise and spikes and false transitions impressed upon the power lines by operation of the thyristors are often introduced into the magnetic field in the alternator through the alternator power output windings. These anomalies are then picked up by the synchronizing windings. Thus, in addition to the expense of a synchronizing winding for each phase, one must add the expense of conditioning circuitry for each of three phases.